Conventionally, a nitride semiconductor light-emitting device constituting a vertical light-emitting diode, a surface-emitting laser, or the like has included a current confinement structure. A vertical nitride semiconductor light-emitting device has a basic structure in which an active layer, a p-type nitride semiconductor layer, and a positive electrode are stacked in that order on an n-type nitride semiconductor layer and in which a negative electrode is fainted on the n-type nitride semiconductor layer. This is a configuration in which light emitted in response to a current being passed from a p-type nitride semiconductor to an n-type nitride semiconductor is extracted toward an upper portion of the stack. Without a current confinement structure, a current in the p-type nitride semiconductor layer flows mainly through a region directly under the positive electrode. This is because a current flows along a shortest current path from the p-type nitride semiconductor to the active layer. Light emitted in the active layer, mainly in the region directly under the positive electrode in which a current is concentrated, is blocked by the positive electrode or absorbed by the positive electrode, and the amount of light that exits to the outside of the vertical nitride semiconductor light-emitting device decreases.
Accordingly, a technique has been devised in which a current path is structurally bent so that a reached position of a current in the active layer may be concentrated at a position shifted from the region directly under an electrode. Such a structure is called a current confinement structure. For example, a configuration is employed in which the positive electrode is formed in the shape of a ring so that a current may be confined inside the ring. Light emitted in the active layer is concentrated inside the ring, and exits to the outside along a central axis of the ring. At this time, the light is not obstructed by the ring-shaped positive electrode, and exits to the outside through the inside of the ring. In this case, a configuration has been proposed in which both of a current path from the positive electrode toward the central axis of the ring and a current path passing through the central axis of the ring and a region therearound toward the active layer have low resistance.
As one example, a nitride-semiconductor light-emitting diode including a buried tunnel junction is described in non-patent document 1. A tunnel junction is a junction relating to a tunnel diode having negative resistance characteristics, which is known as a so-called Esaki diode or the like. With the tunnel junction, unlike rectifying characteristics exhibited by general diodes, a current can be reversely passed from an n layer to a p layer, current-voltage characteristics thereof are ohmic like. For the utilization of the ohmic characteristics in the reverse direction, a tunnel junction having an n-type semiconductor layer on a positive electrode side and a p-type semiconductor layer on a negative electrode side is disposed in a current path inside the ring of the positive electrode. A current is concentrated at the tunnel junction and flows by the tunnel effect. At this time, with the tunnel junction, an n-type semiconductor layer is provided as a layer below the positive electrode instead of the p-type semiconductor layer to reduce the resistance of a current path from the positive electrode toward the central axis of the ring.
The activation of the p-type nitride semiconductor is performed by thermal annealing. Generally, a p-type GaN crystal used as a p-type nitride semiconductor is configured using, for example, Mg atoms or the like as acceptor impurities. Hydrogen atoms produced in a manufacturing process has the property of being taken into the crystal and easily combined with Mg atoms. If Mg atoms are combined with hydrogen atoms, Mg atoms are deactivated and may not function as acceptors. As a result, the GaN crystal may not behave as p-type to lose conductivity and may come to have high resistance. Thermal annealing breaks bonds between hydrogen atoms and Mg atoms and allows hydrogen atoms to exit to the outside of the crystal. This activates Mg atoms as acceptors, and a p-type GaN crystal having conductivity can be obtained. It is mentioned that the thermal annealing is performed in a phase after the formation of the element structure of the light-emitting diode and before the formation of the electrode in the manufacturing process.
Non-Patent Document 1: S. R. Jeon, et al., “GaN tunnel junction as a current aperture in a blue surface-emitting light-emitting diode,” Applied Physics Letter, (U.S.), Jan. 14, 2002, Vol. 80, Number 11, pp. 1933-1935